1. Field of the Invention
The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) catheter for providing energy to biological tissue within a biological site and, more particularly, to an EP catheter having a dielectric-coated ablation electrode having a non-coated window with thermal sensors.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cSA nodexe2x80x9d) located in the wall of the right atrium. The SA node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cAV nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth, remodeling, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the SA and AV nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed percutaneously, a procedure in which a catheter is introduced into the patient through an artery or vein and directed to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or to create a conductive tissue block for preventing propagation of the arrhythmia and restoring normal heart function. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the surgical xe2x80x9cMaze procedurexe2x80x9d involves the formation of continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. Migration to a percutaneous catheter approach removes the morbidity associated with a surgically opened chest cavity. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. In this sense transmural is meant to include lesions that pass through the atrial wall or ventricle wall from the interior surface (endocardium) through the cardiac muscle layer (myocardium) to the exterior surface (epicardium).
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In both the unipolar and the bipolar methods, the current traveling between the electrodes of the catheter and between the electrodes and the backplate enters the tissue and induces a temperature rise in the tissue resulting in ablation.
During ablation, RF energy is applied to the electrodes to raise the temperature of the target tissue to a lethal, non-viable state. In general, the lethal temperature boundary between viable and non-viable tissue is between approximately 45xc2x0 C. to 55xc2x0 C. and more specifically, approximately 48xc2x0 C. Tissue heated to a temperature above 48xc2x0 C. for several seconds becomes permanently non-viable and defines the ablation volume. Tissue adjacent to the electrodes delivering RF energy is heated by resistive heating which is conducted radially outward from the electrode-tissue interface. The goal is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode well below 100xc2x0 C. In clinical applications, the target temperature is set below 65xc2x0 C. to minimize coagulum formation. Lesion size has been demonstrated to be proportional to temperature and duration of ablation.
Blood coagulation is a major limitation/complication associated with RF ablation therapy. Coagulation can lead to thromboembolism and can also form an insulating layer around the electrode hindering further energy delivery required for ablation therapy. Heat appears to be a major factor in the formation of blood coagulum on a catheter electrode. During a typical RF energy ablation procedure using an EP catheter, one or more electrodes carried by the catheter are positioned such that a portion of the electrode(s) are in contact with the tissue being ablated while the remaining portion of the electrodes are in contact with blood. The RF energy applied during the procedure resistively heats the tissue which in turn heats the electrode through conduction. As blood stays in contact with the heated electrode, platelet activation and protein binding occur. This platelet activation appears to be a pathway to coagulum formation.
To reduce the possibility of coagulum formation, one or more thermal sensors may be positioned on the electrodes. Temperature readings provided by the sensors are used to monitor the temperature of the electrodes and to automatically control the power delivered to the electrodes in order to maintain the temperature at or below a target temperature. This type of temperature control scheme assumes that the temperature readings provided by the thermal sensors accurately reflect the temperature at the interface between the electrode and the tissue. This may not, however, be the case, particularly when band electrodes are being used or when thermal sensor orientation to the tissue interface is less than optimum.
During an ablation procedure using a band electrode, only a portion of the band electrode contacts the tissue. Depending on the orientation of the band electrode relative to the tissue and the position of the thermal sensor relative to the band electrode, the thermal sensor may not coincide with that portion of the electrode which contacts the tissue. In this situation, the temperature readings provided by the thermal sensor do not reflect the temperature at the electrode/tissue interface and instead more likely reflect the temperature of the blood pool surrounding the electrode. Power delivery control based on such temperatures may lead to overheating of the electrode/tissue interface and the formation of coagulum.
Hence, those skilled in the art have recognized a need for providing an EP catheter capable of significantly reducing the possibility of coagulum due to electrode overheating regardless of the position of the thermal sensor relative to the tissue. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to an ablation catheter having one or more electrodes partially coated with a dielectric material. The non-coated portion of the electrode defines a window through which ablation energy is transferred. One or more thermal sensors are located within the window to provide temperature readings.
In one aspect, the invention relates to an ablation catheter including a shaft carrying at least one electrode. A thermally conductive and non-electrically conductive surface covering covers a portion of the electrode. The electrode thus has at least one masked, or coated, portion and at least one non-masked, or non-coated, portion. The catheter further includes at least one thermal sensor located in a non-masked portion of the electrode.
The surface covering serves several purposes. It acts as an electrical insulator to prevent alternate or non-intended site ablations, as the electrode only produces a lesion through the non-coated portion of the electrode, i.e., the ablation window. When the electrode is positioned such that RF energy passes through the ablation window to tissue, the surface covering ensures that no RF energy passes through portions of the electrode that do not contain thermal sensors, thereby ensuring that the electrode/tissue interface will possess accurate temperature readings. The surface covering also acts as a thermal conductor to allow for heat to dissipate from the electrode into the surrounding blood pool. The surface covering allows ablation procedures to be performed using less energy since all the power to the electrode is directed through the ablation window, thus minimizing wasted power. Without an ablation window as such, it is possible for the thermal sensors to be misoriented with respect to the actual electrode/tissue interface. If so, then the temperature readings will be lower than the actual interface temperature. This produces ambiguity that is difficult or impossible to resolve and may promote the formation of coagulum.
In a detailed aspect, the surface covering includes a dielectric material. In further detailed aspects, the dielectric material includes one of parylene, polyimide, polytetrafluoroethylene (PTFE), epoxy, polyurethane, polyester and cyanoacrylate and the surface covering has a thickness in the range of approximately 0.001 to 0.05 millimeters. In another detailed facet, the at least one electrode is a band electrode having a width and a circumference and the surface covering covers the width of the electrode and wraps around a portion of the circumference. In a further detailed facet, the surface cover wraps around approximately one-half to three-fourths of the circumference. In another detailed aspect, the thermal sensor comprises a thermocouple having at least two temperature leads electrically coupled to the non-masked portion of the electrode.
In another aspect, the invention relates to a catheter for applying energy to biological tissue having biological fluid flowing thereby. The catheter includes a shaft having a distal-end region defining a tissue-contacting surface and a fluid-contacting surface. A plurality of band electrodes are positioned at the distal-end region of the shaft. A thermally conductive and non-electrically conductive surface covering covers a portion of each of the band electrodes substantially coincident with the fluid-contacting surface. Each band electrode thereby has at least one masked portion substantially coincident with the intended fluid-contacting surface and at least one non-masked portion substantially coincident with the tissue-contacting surface. The catheter also includes a plurality of thermal sensors. At least one thermal sensor is located in a non-masked portion of each of the band electrodes.
In a detailed facet of the invention, a plurality of thermal sensors are located in the non-masked portion of some of the band electrodes. In a more detailed facet there are two thermal sensors located approximately 60 degrees apart along the circumference of the non-masked portion of the band electrode.
In another aspect, the invention relates to an RF ablation system for applying energy to biological tissue having biological fluid flowing thereby. The system includes a catheter having a shaft having a distal-end region defining a tissue-contacting surface and a fluid-contacting surface. The catheter also includes at least one electrode positioned at the distal-end region of the shaft and a thermally conductive and non-electrically conductive surface covering that covers a portion of the electrode substantially coincident with the fluid-contacting surface. The electrode thereby has at least one masked portion substantially coincident with the fluid-contacting surface and at least one non-masked portion substantially coincident with the tissue-contacting surface. The catheter further includes at least one thermal sensor located in a non-masked portion of the electrode and adapted to provide temperature signals indicative of the temperature at the electrode. The system further includes a power generator adapted to provide power to the at least one electrode and a processor adapted to receive the temperature signals from the at least one thermal sensor and control the provision of power by the power generator based on the signals.
In a more detailed aspect, the catheter includes a plurality of electrodes, each having at least one thermal sensor associated therewith, the power generator is adapted to provide power to each of the electrodes based on the temperature signals from that electrode, and the processor is adapted to control the provision of power to each of the electrodes.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.